The present invention relates to electrical converter plants (possibly also including inverters), and in particular to plants comprising at least two converters arranged in parallel between a power supplying line and a power consuming line or load line. Each converter applies a voltage in a narrow predetermined voltage range to the load line, and each converter comprises a control circuit which generates a stabilized output voltage.
Such plants with converters working in parallel on a common load are well known.
Known plants are usually controlled by a common relay-operated control circuit and often work according to a master-slave principle. In this arrangement the master unit is always active, while the slave units, controlled by the control circuit, are switched on/off according to changes in the load.
In such systems the master unit will be exposed to harder working conditions than the slave units. Errors, therefore, more often will occur in the master unit and such errors often will put the whole plant out of operation. Errors in the common control circuit will also be disastrous to the complete system. Another drawback is that if such master-slave systems are to be expanded, a great deal of connecting work has to be done, not only in the new units themselves, but also in the existing "old" system.
A different solution is shown in the U.S. Pat. No. 4,276,590. This patent relates to a modular power system. Each module comprises an inverter and means to determine the current to a common load from the module itself. A difference circuit is also provided to determine the difference between the module supplied current and the average of the currents supplied by the other modules. This difference is utilized to adjust the pulse width of the inverter circuit to equalize the output power.
A system as described in the above mentioned U.S. Patent incorporates only two modules, but can, according to the patent, be expanded by connecting several additional, identical power modules in parallel. Then the additional power modules have to include connections from the corresponding filter network therein, connections to a common synchronizing network and a separate output voltage sensing line.
However, the system has clearly been designed for two modules only. If several modules are connected in parallel, problems will arise due to mutual inter-module currents, and the control sections incorporated in each module must be rather complex and expensive units.
Another drawback with this and similar systems is that the output current and not the output voltage is adjusted. This in spite of the fact that a constant output voltage is desired.